1. Field of the Invention
The present invention relates generally to laser scanners used in reading bar and like code symbols, and more particularly to a novel optical filtering system for use therein, which provides improved scanner performance, appearance and manufacturability at lower cost.
2. Brief Description of the Prior Art
Laser-based bar code symbol scanning systems have become increasingly popular in recent times. However, despite technical advancements in the art, such systems still suffer from numerous problems that have yet to be adequately solved.
For example, a major problem with prior art laser scanners is that as they become more widely used in point-of-sale (POS) environments, aesthetic considerations play a greater role in their purchase decisions by store managers considering their use at POS locations. The reason for this is clear. Store owners invest in a great deal of time, money and artistic effort in making their stores and display counters attractive to customers. Consequently, store owners and managers demand that laser scanning systems do not detract from the appearance of their display and check-out counter environments.
Another problem with prior art laser scanning systems is that the laser, mirrors, and other electro-optical components used in such systems are revealed to customers at POS locations through optically transparent scanning windows. Consequently, the sight of rotating mirrors and swirling laser beams behind the scanning windows of prior art laser scanners, constitutes a significant source of fear to many customers. While such fears are often based on a lack of knowledge of lasers and optics, store managers are nevertheless concerned that such fears may translate into customer anxiety and thus a decrease in sales.
Other problems of a more technical nature arise when using prior art laser scanners in POS environments. In particular, typical ambient lighting levels in store environments have the potential of adversely effecting the signal-to-noise ratio (SNR) of laser scan data signals detected within prior art laser scanners. Thus, to date, a number of different optical filtering techniques have been developed for use in combating the adverse effects of ambient lighting levels on laser scanner performance. Several optical filtering techniques commonly employed are detailed below.
One popular filtering technique involves installing before the scanner photodetector, a band-pass optical filter narrowly tuned to the laser wavelength. Typically, this wavelength lies in the visible region of the electromagnetic spectrum (i.e., about 670 nanometers). This common filtering technique is used in the prior art laser scanning systems disclosed in U.S. Pat. Nos. 5,180,904; 5,015,833; 4,816,660; 4,387,297 and 5,115,333. However, this approach is not without shortcomings and drawbacks. When using this approach, store customers are typically permitted to see the rotating or oscillating mirrors and swirling laser beams behind the scanning window. In addition to presenting a source of worry for many customers, the plain view of such electro-optical components also detracts from the overall aesthetic appearance of laser scanners employing this common filtering technique.
Another prior art approach to reducing ambient light in a post-based laser scanners involves installing a spatial filter (i.e., a slotted or aperture plate) over the scanning window of the laser scanner. Typically, the aperture or slot pattern of the aperture plate spatially corresponds to the cross-sectional geometry of projected laser scanning pattern at the plane of its scanning window. This spatial filtering technique is used in the many prior art laser scanning systems, disclosed in U.S. Pat. Nos. 4,713,532; 4,093,865; and 4,647,143. However, this approach is not without its shortcomings and drawbacks. Such spatial filters detract from the overall appearance of the laser scanners in which they are employed. In addition, such spatial filters cannot be effectively used when the laser scanning patterns are spatially complex, as in the case of the omni-directional projection laser scanner disclosed in U.S. Pat. No. 5,216,232.
Thus, there is a great need in the art for a laser scanner which solves the above-described problems, while overcoming the shortcomings and drawbacks of prior art laser scanning apparatus and methodologies.